This invention relates to an improved OTP (One-Time Programmable) memory cell and, in particular, a three-transistor OTP memory cell with improved programmability.
NVM (Nonvolatile memory) integrated circuits retain stored data even after power is removed. Such devices are found in many consumer electronics and industrial applications. A common memory cell in NVM integrated circuits is the antifuse memory cell based upon the programming of a gate dielectric layer. Such antifuse memory cells, more particularly, the one-time programmable (OTP) memory cell, are commonly used by integrated circuit designers since such cells are compatible with standard logic CMOS (Complementary Metal-Oxide-Semiconductor) process flows.
An OTP memory cell includes a dielectric layer, such as the gate oxide, in a capacitor structure. The memory cell may include one or more select devices, such as a MOSFET (Metal-Oxide-Semiconductor-Field-Effect-Transistors), or a diode, such as a gated diode, to enable selective programming, the rupturing of the dielectric layer for an electrical connection, and reading of the memory cell. Each memory cell stores a bit of information, a “1” or a “0,” depending on whether the cell has been programmed or not. A correspondence between a value of a bit and whether a cell is programmed may be defined arbitrarily. An array of such OTP memory cell includes the location of memory cells at cross points of orthogonal bit lines and word lines.
Even though OTP memory cell arrays have been in use for many years, a persistent problem is the variability in the quality of the programmed connection in a memory cell. A significant portion of this variability can be attributed to layout-dependent effects on the program capacitor and the select device. That is, physical variations in the OTP memory cells adversely affect the post-programmed resistance and therefore the reading of the OTP memory cell state. A more robust antifuse OTP design is desired to improve the physical uniformity of the array memory cells and the distribution of the programmed cell read current.